Does genomic evidence make it scientifically impossible that the human lineage could have ever passed through a population bottleneck of just two individuals? This is a question I am asked semi-frequently by religious friends. With my current understanding of the genetic evidence, I can’t state categorically that it’s impossible. In this view, I find I differ from a recent book chapter on the topic. I’m writing this blog to run my thoughts past other biologists, and check I am not missing something.

One needn’t be religious to see the importance of this question. If one pair of any sexually dimorphic life form might pass successfully through a bottleneck, ecologists need to take that fact into account.

It is a bit of a leap to say that if humans had passed through a bottleneck they would have similar levels of genetic diversity to Tasmanian devils. Whilst it seems clear that the Tasmanian devils have passed through a bottleneck, not all bottlenecks are the same. They can differ in their length as well as in their intensity, and a short bottleneck has less severe consequences than a long one. A short, sharp bottleneck is all that is needed for the Adam and Eve hypothesis.

It is easy to have misleading intuitions about the population genetic effects of a short, sudden bottleneck. For example, Ernst Mayr suggested that many species had passed through extreme bottlenecks in founder events. He argued that extreme loss of diversity in such events would promote evolutionary change. … More.

Um, what is the ALTERNATIVE to a male human and a female human mating to produce the first viable human baby?

As near as I can see, the alternative MUST be that Adam boffed chimpanzees in the absence of Eve. And of course based on Soviet experiments in the 1930s, we know that human-chimp matings do not produced viable offspring.

So, Eve MUST HAVE been born while Adam was still alive (or Adam born while Eve was still fertile). And of course the most likely parents of Eve would have been the same pair of chimps that produced Adam.

A scenario where chimps (or the much sought for and never found Missing Link) produce a whole mess of quasi-human children IN THE SAME GENERATION sounds a WHOLE LOT like a biology experiment by Space Aliens.

But then none of the other Just So stories about the ancestry of whales or bats or even horses hold water either. New species appear POOF! And there is never any transitional animal. So, yes, I agree with the “alien biology experiment” theory. I just believe that the Alien designed the new product lines before the first new models appeared in the show room.

Um, what is the ALTERNATIVE to a male human and a female human mating to produce the first viable human baby?

Something more complicated?

A similar problem would be to ask “who was the first American?” Was north America only colonised by a single couple? If it wasn’t (i.e. if it was a group of people), how do you say who was the first American?

The following quote from article published in Nature which examined several scientific studies that looked into genetic variation as it pertains to race.

The picture that begins to emerge from this and other analyses of human genetic variation is that variation tends to be geographically structured, such that most individuals from the same geographic region will be more similar to one another than to individuals from a distant region. Because of a history of extensive migration and gene flow, however, human genetic variation tends to be distributed in a continuous fashion and seldom has marked geographic discontinuities. Thus, populations are never ‘pure’ in a genetic sense, and definite boundaries between individuals or populations (e.g., ‘races’) will be necessarily somewhat inaccurate and arbitrary.

Though now supported by many genetic data, this concept is hardly new. Blumenbach, writing in the 1700s, acknowledged extensive morphological overlap among populations or races. Charles Darwin, some 100 years later, wrote, “It may be doubted whether any character can be named which is distinctive of a race and is constant.” Scientists recognized shared genetic variation among populations more than 50 years ago, although their conclusions were based on relatively small numbers of informative loci. This pattern of shared variation has important implications for our understanding of population differences and similarities, and it also bears on critical biomedical issues.

The entire problem here has to do with how effective population sizes are calculated. It is the the harmonic sum of the population size for each generation, divided by the number of generations, which is then inverted. This is no more than an approximate method that is not outside of criticism.

But criticism aside, if you have two individuals who give rise to 8 progeny, who pair off, and each couple has 8 progeny, then within 8 generations you have a population size of over 34,000 individuals. If that population size is maintained for the next 5,000 generations (approx. 100,000 years for humans), then the ‘effective population size’ is calculated to be 6,000.

In the comment posted at Nature, Buggs writes:

This is a rather inaccurate presentation of the paper. The paper’s discussion starts with the sentence: “Overall, the estimates of Ne appear to be much lower than the usually quoted value of 10,000”. It seems to me that the paper gives no warrant for Venema’s addition of effective population sizes above and below the Sahara, as it explains that the non-sub-Saharan populations contain a “subset of the amount of genetic variation present in the African population” due to an out-of-Africa expansion. It is the ancestral sub-Saharan estimate that therefore of main interest to us. The mean estimated Ne for this population among chromosomes is 6286, with a standard deviation of 1357.

Interestingly, if you build up to a population of 8,000, and let that remain constant for 250,000 years—which mimics some of the numbers used in the study Buggs, and Venema, refer to, then you get an effective population size of 5,714—at the low end of the range given above in the blockquote.

I doubt anyone is demonizing for your whiteness, but doesn’t the fact there are no substantial biological differences among culturally-constructed races make you stop and think about the privileges associated with being white in the US. If the severe racial inequalities that exist cannot be explained by biology, what can explain them?

As improbable as it might seem that all humanity descended from a single pair, that is exponentially more likely than for a prokaryote to evolve to a eukaryote, which is the dogma demanded by the faith of Darwin.